1. Field of the Invention
The invention relates to a plasma display device, and a method of reducing interference to radio-broadcasting waves, caused by electromagnetic waves derived from a plasma display device.
2. Description of the Related Art
A plasma display device displays imaged by radiating ultra-violet ray generated by gas discharged to fluorescent material to thereby excite the fluorescent material and cause the fluorescent material to emit light therefrom. A plasma display device is grouped into AC type in which an AC (Alternating Current) power supply is used and DC type in which a DC (Direct Current) power supply is used.
An AC type plasma display device is superior to a DC type one in brightness and a light-emission efficiency, and is simpler in structure and easier to be fabricated in a large size than a DC type one. Hence, an AC type plasma display device is used more broadly than a DC type one.
An AC type plasma display device is further grouped into various structures. Among them, a three-electrode surface-discharge type AC plasma display panel is in particular broadly used because of its long lifetime.
FIG. 1 illustrates an example of a partial structure of a three-electrode surface-discharge type AC plasma display panel.
A three-electrode surface-discharge type AC plasma display panel is comprised of a front substrate located closer to a viewer and a rear substrate located remoter from a viewer. The front substrate includes a plurality of sustaining electrodes (common electrodes) 101, and a plurality of scanning electrodes 102 both formed on a surface of the front substrate facing the rear substrate. A sustaining electrode 101 and a scanning electrode 102 located adjacent to the sustaining electrode 101 make a row-electrode pair acting as a one display-row. The rear substrate includes a plurality of column electrodes (address electrodes or data electrodes) formed on a surface thereof facing the front substrate. The column electrodes (not illustrated) extend perpendicularly to the row-electrode pair.
A plurality of display cells 103 are defined at intersections of the sustaining electrode 101 and the scanning electrode 102 with the column electrodes.
By applying a drive voltage across the column electrode and the scanning electrode 102, there is generated writing-discharge therebetween for selecting a display cell 103 from which light is emitted. Further, by applying a drive voltage between the sustaining electrode 101 and the scanning electrode 102 both of which make a column-electrode pair, there is generated sustaining-discharge comprised of surface-discharge in the selected display cell 103.
In a three-electrode surface-discharge type AC plasma display panel, highly energized ions generated when the surface-discharge is generated at the front substrate do not bombard and hence degrade a fluorescent material layer formed on the rear substrate, and hence, ensuring that a three-electrode surface-discharge type AC plasma display panel can have long lifetime.
In a three-electrode surface-discharge type AC plasma display panel, when a sustaining discharge (light-emission discharge) is to be generated, that is, during a sustaining-drive period, a high-frequency pulse or a drive pulse having a 100 to 500 KHz is applied across entirety of the sustaining electrodes 101 and the scanning electrodes 102.
As illustrated in FIG. 1, the sustaining electrodes 101 are electrically connected to one another through a bus electrode 104a extending along a left edge of a plasma display panel, and similarly, the scanning electrodes 102 are electrically connected to one another through a bus electrode 104b extending along a right edge of a plasma display panel.
Hence, a current caused by sustaining-drive and light-emission discharges flows at a time at one of the left and right edges of a plasma display panel.
FIG. 2 is an example of a cross-sectional view of a plasma display device.
As illustrated in FIG. 2, a first drive circuit 111 and a second drive circuit 112 are arranged at the rear of the plasma display panel 110. A ground (GND) plate 113 is sandwiched between the plasma display panel 110 and the first and second drive circuits 111 and 112.
In the plasma display panel 110 having the above-mentioned structure, when sustaining-drive and light-emission discharges are generated, a current flows in a forward direction A or a reverse direction B in a route defined by the first drive circuit 111, the ground plate 113, the second drive circuit 112 and the plasma display panel 110 connected to one another is this order. That is, when sustaining-drive and light-emission discharges are generated, there is formed a current loop 115 extending along the route, and the current loop 115 defines a loop antenna.
A current having a broadband frequency flowing through the loop antenna would cause quite intensive broadband electromagnetic radiation. Accordingly, it is necessary for a plasma display device to have an electromagnetic shield at an outer surface thereof for reducing such disadvantageous electromagnetic radiation.
Herein, attention is paid in particular to a relatively low frequency of a medium-wave band. A total length of the loop antenna is sufficiently shorter than a wavelength of a wavelength of such a medium-wave band. Hence, electromagnetic radiation caused when a high-frequency current flows in the loop antenna would have a property of magnetic field source, that is, leaked magnetic flux. Among leaked magnetic flux, a most intensive one is generated by drive pulses applied for generating light-emission discharge.
Since a plasma display device is required to be thin, it is necessary for such an electromagnetic shield as mentioned above to be arranged in the vicinity of a source of electromagnetic radiation. However, it is well known that a shield to a source of electromagnetic radiation is more difficult to fabricate, if the electromagnetic radiation has a lower frequency, or the shield has to be arranged closer to a source of electromagnetic radiation.
The shield is required to have superior performance. In order to accomplish such a shield having superior performance, it would be necessary for the shield to be composed of metal having high magnetic permeability and further having a sufficient thickness. However, a plasma display device including such a shield would be heavy and large in a size.
In particular, an optical filter attached to a surface of a display screen in a plasma display panel is required to have optical permeability and magnetic shield performance, resulting in that an optical filter would be very expensive.
For the reasons mentioned above, a conventional plasma display device cannot sufficiently reduce leaked magnetic flux in a medium-wave frequency band.
A so-called AM radio which receives medium radio-broadcasting waves is usually designed to have receipt sensitivity by means of a bar antenna composed of ferrite, and hence, is more likely to be influenced by induction of magnetic flux than electric field. Accordingly, when a user listens to an AM radio, if a frequency of radio-broadcasting waves is coincident or approximately coincident with any one of harmonics among frequencies of sustaining-drive pulses in a plasma display panel operating in the vicinity of the AM radio, the AM radio would be interfered by intensive electromagnetic waves derived from the plasma display panel. This results in that a user cannot properly listen to the AM radio.
That is, harmonics generated when a plasma display device is driven might cause electromagnetic interference to receipt of medium radio-casting waves. Such electromagnetic interference is caused not only in a house, but also in a plurality of houses such as a condominium in one of which a plasma display device is used.
A plasma display device is now broadly used, and accordingly, it is required that electromagnetic environment around a house in which a plasma display device is used is not deteriorated, and there is not caused electromagnetic interference to receipt of AM radio-broadcasting waves.
In order to meet such requirement, Japanese Patent Application Publication No. 2000-338932 has suggested a plasma display panel in which a frequency of a driving pulse, that is, a sustaining-drive frequency is successively varied with the lapse of time to prevent spectrum of radiation noise from concentrating to a specific frequency.
However, the suggested plasma display panel is accompanied with a problem as follows because of successive variance of a frequency of a drive pulse or successive modulation of a frequency of a drive pulse.
If a frequency of a driving pulse is successively varied with the lapse of time, harmonics having a common frequency are varied with the lapse of time together with the variance of the frequency of a driving pulse.
Hence, a condition in which a frequency of radio-broadcasting waves is coincident with any one of harmonics when a user listens to an AM radio in the vicinity of a plasma display panel, and a condition in which the frequency is not coincident with any one of harmonics are repeatedly caused with the lapse of time.
Thus, the plasma display panel suggested in the above-mentioned Publication can reduce interference to AM radio-broadcasting waves, but cannot completely prevent such interference.
For instance, in Japan, frequencies are applied to medium AM radio-broadcasting waves by every 9 KHz. An AM radio generally has a band-width of 10 KHz or greater for receiving radio-broadcasting waves. However, the plasma display panel suggested in the above-mentioned Publication periodically varies the sustaining-drive frequency within a limited range around a predetermined fixed frequency, and hence, there exist a plurality of radio-broadcasting waves which cause interference, in any one of regions in Japan.
Furthermore, harmonics radiated from a plasma display device would have an extended band-width due to the frequency modulation in the plasma display panel suggested in the above-mentioned Publication. Accordingly, possibility at which interference is caused to any one of AM radios broadcasting at various frequencies is increased.